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Western populations average slightly smaller (Wetmore 1940c, Wetmore 1949), but adult male plumage is essentially invariant geographically (Wetmore 1949). Females and young males vary in dorsal coloration, with western and northern breeders having the back and crown brownish, eastern and southern breeders having the back greenish and crown grayish, and Newfoundland breeders having the back and crown grayish (Browning 2002).
Two subspecies were diagnosed on the basis of dorsal color of the female and immature male (Wetmore 1949). However, recognition of subspecies is disputed. Some ornithologists have suggested that northern breeders—essentially those occurring across Canada and into the northwestern U.S.—differ markedly from southern and eastern breeders (Oberholser 1938, Burleigh and Peters 1948). Yet others have been unable to diagnose populations (e.g., Wetmore 1940c), and many subsequent authors have either sidestepped potential differences (e.g., Mengel 1965b) or simply treated the species as monotypic (e.g., Mayr and Short 1970, Gibson and Withrow 2015a). Nevertheless, after further assessment of variation in female coloration, Wetmore 1949 concluded that his earlier assertion of no subspecies had been hasty. Browning 2002 further noted that there are more likely to be three subspecies than two, let alone none. The American Ornithologists' Union committee of classification and nomenclature, of which Wetmore was chair, recognized two subspecies (American Ornithologists' Union 1949, American Ornithologists' Union 1957). Differences in mitochondrial DNA are slight across the species’ breeding range, although there is evidence for genetic isolation by distance, which implies limited genetic dispersal today, primarily among females (Colbeck et al. 2008). Future genetic work is needed to identify possible geographic variation in traits other than just plumage (see Priorities for Future Research).
S. r. ruticilla Linnaeus 1758. Includes Motacilla flavicauda Gmelin 1788, and M. multicolor Gmelin 1788. Breeds in the eastern U.S. through the Appalachians and through the Mississippi Valley [type locality = Virginia]. Dorsum of female and immature male paler gray, with the back brighter green.
S. r. tricolora Müller 1776. Breeds across Canada and into the northwestern U.S., south sporadically into northernmost California and into the northern Rocky Mountains, and in northern New England [type locality = Cayenne, French Guiana]. Dorsum of female and immature male darker gray, with the back duller green.
The American wood-warblers (Parulidae) are a key component in a broad and, in geological terms, recent radiation of passerines with nine primaries that also includes the families Emberizidae, Cardinalidae, Thraupidae, and Icteridae (Klicka et al. 2007). Within the Parulidae, comprehensive genetic research has reestablished generic relationships (Lovette et al. 2010; see also Lovette and Hochachka 2006). A key finding was that the genus Dendroica is paraphyletic, and includes Wilsonia citrina (the Hooded Warbler) and S. ruticilla (the American Redstart). As a result, all species of Dendroica warblers, as well as W. citrina, were merged into the genus Setophaga (Chesser et al. 2011), the oldest genus and one that for many decades was thought to be monotypic. Hence, Setophaga went from being monotypic to the most speciose genus (34 species) in the family, easily surpassing the Neotropical genus Basileuterus (26 species).
The nearest relative of S. ruticilla is unclear (Lovette et al. 2010). The species’ song resembles that of S. petechia (the Yellow Warbler) and S. pensylvanica (the Chestnut-sided Warbler), and it resembles various other species in the genus in terms of its eggs, nest, plumage pattern, development, and agonistic and courtship behaviors (Parkes 1961, Ficken and Ficken 1962b, Ficken and Ficken 1965). S. ruticilla has hybridized with S. americana, the Northern Parula (Burleigh 1944a, Bent 1953b), S. ruficapilla, the Nashville Warbler (Dunn and Garrett 1997), and, apparently, Geothlypis philadelphia, the Mourning Warbler (McCarthy 2006).